refactor(source): Move lua file detection to do_source

So now :source can run lua files too :)

* feat: Add support for :[ranged]source for lua files

test/functional/ex_cmds/source_spec.lua

@@ -6,6 +6,9 @@ local clear = helpers.clear
 local meths = helpers.meths
 local feed = helpers.feed
 local feed_command = helpers.feed_command
+local write_file = helpers.write_file
+local exec = helpers.exec
+local eval = helpers.eval
 
 describe(':source', function()
   before_each(function()
@@ -44,4 +47,47 @@ describe(':source', function()
     command('source')
     eq('4', meths.exec('echo luaeval("y")', true))
   end)
+
+  it('can source lua files', function()
+    local test_file = 'test.lua'
+    write_file (test_file, [[vim.g.sourced_lua = 1]])
+
+    exec('source ' .. test_file)
+
+    eq(1, eval('g:sourced_lua'))
+    os.remove(test_file)
+  end)
+
+  it('can source selected region in lua file', function()
+    local test_file = 'test.lua'
+
+    write_file (test_file, [[
+      vim.g.b = 5
+      vim.g.b = 6
+      vim.g.b = 7
+    ]])
+
+    command('edit '..test_file)
+    feed('ggjV')
+    feed_command(':source')
+
+    eq(6, eval('g:b'))
+    os.remove(test_file)
+  end)
+
+  it('can source current lua buffer without argument', function()
+    local test_file = 'test.lua'
+
+    write_file (test_file, [[
+      vim.g.c = 10
+      vim.g.c = 11
+      vim.g.c = 12
+    ]])
+
+    command('edit '..test_file)
+    feed_command(':source')
+
+    eq(12, eval('g:c'))
+    os.remove(test_file)
+  end)
 end)